The present invention relates to a fuse for the push rod of a pneumatic brake booster, to its method of assembly and to the pneumatic brake booster comprising such a fuse.
Pneumatic braking installations comprising a pneumatic brake booster are known. Such a pneumatic booster is described for example in document FR-A-2 744 085 in the name of the applicant company. It comprises a front shell and a rear shell together forming a rigid casing. A moving partition sealingly divides the inside of this rigid casing and therein delimits a front chamber and a rear chamber. At rest, the front chamber is connected to a source of depression which delivers a pressure Pd that is low by comparison with atmospheric pressure Pa. At rest, the rear chamber is in communication with the front chamber. A pneumatic piston moves with the moving partition. The booster is controlled by a control rod that can move between a rest position and an actuating position. When a force is exerted on the control rod by the brake pedal, this force, higher than the return force of a spring, causes the communication between the front and rear chambers to be closed, causes the rear chamber to communicate with an environment at atmospheric pressure, and pushes the moving partition forward and generates a boost force used to actuate a master cylinder.
The booster also comprises a reaction disk arranged in front of a plunger controlled by the control rod and housed in a housing borne by one end of a push rod transmitting the boost force to a piston of a master cylinder. The reaction disk is made of substantially incompressible materials combining a control force applied by the plunger, a boost force applied by the pneumatic piston and a reaction force from the master cylinder and transmitted via the push rod, and allowing the driver to feel, at the brake pedal, the reaction of the brake circuit and thus adapt his braking.
In these pneumatic installations fitted to motor vehicles, the control rod actuated by the brake pedal is connected to the master cylinder. When the motor vehicle is in a frontal impact with an obstacle, it has been found that the engine and battery assembly located in the engine compartment is pushed against the bulk head that separates the engine compartment and the cabin, and the thrust experienced by the engine assembly is passed on to the pneumatic brake booster, and therefore to the master cylinder and to the pneumatic brake booster as a whole. As there is a connection between the driver""s foot resting on the brake pedal and the end of the master cylinder, the rearward thrust due to the impact is passed on to the driver""s foot, which suffers an impact.
The object of the invention is therefore to eliminate the impact suffered by the driver""s foot when the motor vehicle is in collision with an obstacle.
It is known that, in order to eliminate this impact, it is necessary, in order to prevent the return of the pedal, to cut part of the transmission between the brake pedal connected to the control rod of the pneumatic brake booster and the master cylinder. For this, a fuse device has been provided in the pneumatic brake booster which eliminates the kinematic connection between the brake pedal and the master cylinder. This fuse device consists of a ring which holds the end face of the push rod of the piston and the housing of the reaction disk. This ring is held by compression on the push rod and the housing or by crimping to the periphery of the push rod and of one end of the housing. It is therefore external to the primary piston rod. One disadvantage with such a system is therefore its axial bulk. If there is a sufficient impact, the ring deforms under the action of sufficient torque and frees the two components.
Another object of the invention is therefore to eliminate the radial bulk of the known fuse device.
Yet another object of the invention is to eliminate the ring and therefore to reduce the number of parts of which the pneumatic brake booster is made.
Another object is to reduce the cost of the pneumatic brake booster, this cost being reduced by eliminating a part and by simplifying the assembly.
To this end, the invention relates to a fuse device for a pneumatic brake booster, characterized in that it comprises a hollow cylindrical first part one end of which has an inside diameter d1 and a bowl-shaped second part with a circular bottom equipped on its central external part with a roughly cylindrical stud of an outside diameter d2 slightly greater, to within the tolerances, than the inside diameter d1 of said end of said hollow cylindrical first part, so as to allow said hollow cylindrical first part to be secured to said bowl-shaped second part by forcibly inserting said stud in said hollow end of smaller diameter d1, said bowl-shaped second part having a circular external groove whose purpose is to weaken the connection between said stud and said bottom of said bowl-shaped second part when a pair of forces is applied to said fuse device, so as to allow said hollow cylindrical first part to detach from said bowl-shaped second part.
The fuse device for a booster is also such that said hollow cylindrical first part is intended to form the push rod of the pneumatic brake booster.
Said bowl-shaped second part is intended to receive a reaction disk made of an elastomer placed at the bottom of the bowl, the reaction disk fitting the booster boost piston body.
Another subject of the invention is a pneumatic brake booster comprising: a rigid casing, a moving partition sealingly defining a front chamber and a rear chamber inside the casing, the front chamber in operation being subjected to a first pressure and the rear chamber being connected selectively to the front chamber or subjected to a second pressure higher than the first, a pneumatic piston moving with the moving partition, an axial control rod moving toward the piston as a function of an input force selectively exerted against the action of a return force and directed in an axial direction of actuation pointing toward the front chamber, the return force urging the control rod toward a return position and the input force urging the control rod 8 toward an actuating position which is dependent upon the input force, a three-way valve connecting the rear chamber to the front chamber when the control rod is in the rest position, and subjecting the rear chamber to the second pressure when it is actuated by movement of the control rod toward its actuated position, and a push rod, characterized in that it comprises a fuse device defined hereinabove.